docs: def get_block(self, usage_id, for_parent=None):
          """
          Create an XBlock instance in this runtime.
  
          The `usage_id` is used to find the XBlock class and data.
          """
          def_id = self.id_reader.get_definition_id(usage_id)
          try:
              block_type = self.id_reader.get_block_type(def_id)
          except NoSuchDefinition:
              raise NoSuchUsage(repr(usage_id))
          keys = ScopeIds(self.user_id, block_type, def_id, usage_id)
          block = self.construct_xblock(block_type, keys, for_parent=for_parent)
          return block